A hydrostatic transmission for automotive vehicle and other load-operating applications can comprise a hydraulic (hydrostatic) motor connected to a load such as the driving wheels of a vehicle, a variable-displacement reversible hydraulic (hydrostatic) pump connected to a prime mover such as the internal combustion engine of the vehicle, a hydraulic circuit connecting the pump and the motor so that the direction of rotation of the motor is a function of displacement of a control element of the pump to one side or to the other side of a null or neutral position and the speed of the motor is a function, for a given speed of the prime mover, of the degree of displacement of the control element from its null position, and a piston-and-cylinder arrangement for actuating the control element.
Frequently a control-pressure pump is also connected to the prime mover for developing the fluid pressure of a control medium which is applied to one side (compartment) or the other side (compartment) of the latter arrangement to displace the movable member thereof from its null position one or the other direction.
The movable member is generally spring centered in its null position and means can be provided for controlling the pressure in the two compartments, e.g. to selectively reduce the control pressure in one compartment, to effect displacement of the control member of the piston and cylinder arrangement which thus functions as a servocylinder, control cylinder or regulating cylinder.
In German patent document (open application) DE-OS No. 26 01 226, the control piston has a rod enabling manual operation which is difficult to control.
In practice, moreover, high pedal forces are found to be required to operate the control cylinder so that the control mechanism suffers undue wear. In addition, the pedal mechanism is subject to failure which makes emergency stops impossible.
Finally, extremely fine control of the output of the transmission is not possible with this earlier system especially if the prime mover or engine is operating at high speeds which would result in high fluid displacement by the pump upon movement of the control element for its null position. Such fine control of the transmission is a requirement for many types of vehicles, for example so-called industrial floor vehicles, e.g. fork lift trucks which frequently must be maneuvered very slowly to bring them into load-pickup and load-depositing positions.